1. Field of the Invention
The present invention relates to an optical device and a Fresnel lens sheet comprising it.
2. Description of the Related Art
FIG. 10 is a schematic view showing the outline of the constitution of a conventional rear projection display. As illustrated, the conventional rear projection display comprises a projection tube 11 which projects an optical image, a projection lens 12 which magnifies the optical image from the projection tube 11, and a rear projection screen 13 which forms thereon the optical image having been magnified through the projection lens 12. A person P views the optical image having been projected, magnified and formed on the rear projection screen 13.
In many cases of the rear projection display of that type, the screen 13 has a two-sheet structure composed of a Fresnel lens sheet 14 which gathers the beams of light from the projection tube 11 toward the viewer, and a lenticular lens sheet 15 which disperses the light having passed through the Fresnel lens sheet 14 in the direction horizontal to the screen (that is, in the direction of the width of the screen) and in the direction vertical thereto (that is, in the direction of the height of the screen) at a predetermined angle in a suitable ratio to thereby enlarge the angle of visibility to be in a predetermined range.
In that type of rear projection display equipped with a rear projection screen, the projection lens 12 and the screen 13 are desired to be so disposed that the distance therebetween is shorter so as to reduce the thickness of the display. It is also desired to increase the luminance around the peripheral area of the screen. To meet the requirements, the Fresnel angle in the peripheral area of the Fresnel lens sheet 14 must be enlarged. On the other hand, in general, the refractive index of the material that forms the Fresnel lens surface of the Fresnel lens sheet has wavelength dependency. Therefore, the angle of light passing through the Fresnel lens shall vary, depending on the wavelength of the light. With the Fresnel angle increasing, the difference in the angle of light passing through the Fresnel lens shall increase, depending on the difference in the wavelength of the light. As a result, the light having reached the screen is colored to degrade the quality of the image formed on the screen. Accordingly, it is difficult to solve the problem with thinned rear projection displays only by enlarging the Fresnel angle in the peripheral area of the Fresnel lens sheet.